This invention relates to bearing assemblies. In particular, the present invention is a bearing assembly having thermal compensation connectors that permit expansion and contraction, due to temperature variations, of bearing assembly components having different coefficients of thermal expansion. The invention described herein was made in the performance of work under NASA Contract No. NAS9-18200 and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 (42 U.S.C. 2457). The invention described herein has been granted NASA Waiver Case No. W-2975.
In the aviation industry and the space industry, it is critical that aircraft and spacecraft systems be manufactured to be as lightweight and durable as possible. In particular with regard to spacecraft, it is essential that lightweight materials be used to manufacture spacecraft components of spacecraft systems, since each incremental increase in spacecraft component weight requires an incremental increase in the amount of rocket fuel required to launch the spacecraft into outer space. Because spacecraft components of spacecraft systems are submitted to high structural stresses and extreme temperature fluctuations within the range of 180.degree. F. to -65.degree. F., it is also essential that spacecraft components be manufactured of materials that are durable. However, it is not always possible to manufacture spacecraft components of a spacecraft system to a desired weight and a desired durability using a single material. Hence, spacecraft systems often are comprised of spacecraft components manufactured of various materials, with the choice of a particular material being dependent upon the purpose served by the particular spacecraft component. For example, spacecraft components subjected to high structural stresses, such as bearings, are typically manufactured from heavy, structurally strong and durable materials, while spacecraft components that are not subjected to extreme structural stresses, such as stator and rotor housings, are typically manufactured from lightweight and less durable materials.
One concern of tailoring the material to the purpose of the spacecraft component is that different materials exhibit different physical properties. One of these physical properties that can vary from one material to the next is the coefficient of thermal expansion. When inner and outer parts of different materials having different coefficients of thermal expansion are fitted together at one temperature (the material of the inner pan having a smaller coefficient of thermal expansion than the material of the outer part) then brought to a cooler temperature, a contact pressure is created between the inner and outer parts due to the outer part shrinking faster than the inner part. This contact pressure creates a compressive stress in the inner part material and a tensile stress in the outer part material. These stresses are undesirable and can cause part failure or deformation which may affect the operation of the parts. If the inner and outer parts are brought to a temperature warmer than the temperature at which the pans were initially assembled, a gap will form between the inner and outer parts, due to the outer part growing faster than the inner part. This gap is undesirable since the inner part is no longer held securely by the outer part.
This undesirable contact pressure/gap is especially acute in the spacecraft environment, and in particular, spacecraft bearing assemblies. Typically, a spacecraft bearing assembly is defined by a bearing mechanism, an inner housing and an outer housing. The bearing mechanism is made of steel (a durable, we are resistant material), and is fitted between the inner and outer housings, which are made of aluminum (a lightweight material). The steel of the bearing mechanism has a coefficient of thermal expansion of 6.5.times.10.sup.-6 in./in.-.degree. F., while the aluminum of the inner and outer housings has a coefficient of thermal expansion of 12.4.times.10.sup.-6 in./in.-.degree. F. Typically, the housings and bearing mechanism are press fit together at one temperature (i.e., room temperature within the range of 60.degree. F. to 80.degree. F.). In the environment of space, the housings and bearing mechanism, are subjected to temperature variations within the range of 180.degree. F. to -65.degree. F. These temperature variations, together with the differing coefficients of thermal expansion of steel and aluminum, cause the housings and bearing mechanism to expand and contract at different rates. In turn, this differential expansion and contraction causes contact pressure/gaps between the bearing mechanism and the housings, which results in large bearing stresses and high drag torque and ultimately shorter bearing life.
There is a need for improved bearing assemblies. In particular, there is a need for a bearing assembly with thermal compensation that will virtually eliminate bearing stresses due to the affects of extreme temperature fluctuations on the materials (having differing coefficients of thermal expansion) of the bearing assembly components. The bearing assembly should provide thermal compensation while maintaining a weight efficient structure.